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All I can find online are descriptions on how active buzzers have an oscillator built in so that you just need to feed them a DC and they buzz at the frequency set by such oscillator, but I haven't been able to find either a diagram, description or teardown of an active buzzer to see how is this usually done.

I can easily see how to build an active buzzer from a passive buzzer providing one of infinite possible ways of implementing an oscillator, but I wonder how is it actually done, specially on the tiny packages.

I've seen a teardown here, fantastic BTW, but I assume there are other much simpler/cheaper constructions.

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There are 2 kinds of buzzers.

  1. Piezoelectric ; which use internal CMOS Osc like 4xxx These operate like any Crystal Osc. Except the ceramic diaphragm is a large crystal. All std ceramic caps in fact are piezoelectric but incapsulated. These buzzers operate over a wide V+ range and use low current.

Schmitt Trigger Inverter oscillators are very simple using Dc negative feedback to self bias and with 180 deg phase shift , Ac Positive feedback to oscillate reliably with a square wave. An extra inverter doubles the voltage across the resonator.

Since they use 4xxx series CMOS the driver impedance fairly high from 3k to 300 ohms at max voltage, they may be ganged in parallel for higher power.

  1. Magnetic ; which use a relay type mechanism on the sound diaphragm that the internal coil current breaks from the metallic diaphragm , when actuated , so it relaxes by a spring and repeats . These take more current >20 mA typ depending on dBa levels.
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The method you are looking for includes the piezo element in the feedback path of an oscillator circuit. In this way the beeper always is run at its resonant frequency, despite part-to-part variations, aging, etc.

The most common way in a packaged unit is a 3-terminal piezo element and a 1-transistor circuit. Some piezo element datasheets and catalogs have a version of this in the applications section. Here is an example from somewhere else on Stack Exchange about 4 years ago:

enter image description here

A relatively famous circuit was published in Electronic Design waaay back. It uses a hex inverter as the inverting amplifier, and more sections of the hex inverter as drivers for the element. The section in the upper left corner is a low frequency oscillator that makes the beeper pulse about once per second:

enter image description here

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